1. Technical Field
The present disclosure relates to photovoltaics, and more specifically to photovoltaics with transparent conductive coatings.
2. Description of Related Art
Concentrating photovoltaics (CPV) have great potential to make solar energy cost competitive with other energy sources. In CPV, sunlight is collected from a large area and concentrated on a relatively small solar cell area through the use of some combination of reflective and/or refractive optics. The solar cell comprises semiconductors that convert sunlight into electricity. Concentration of the sunlight increases the efficiency of the solar cells and reduces the volume of (expensive) solar cell material required to produce a given power output. In such a system, very high efficiency (and high cost) multijunction cells may be used with a net improvement in overall system cost per unit energy.
The high current densities generated in the semiconductor under concentration can, however, result in significant resistive power losses that can offset the gains in efficiency brought by concentration. With multijunction cells, thick, closely spaced gridlines of silver or another highly conductive material may be provided on the sunward surface of the cell to collect the current. For example, FIG. 1 illustrates an example of a semiconductor wafer 10 having a network of gridlines 12 on the sunward surface 14 thereof. Sunlight 16 reaches the semiconductor 10 in the regions not obscured by the gridlines. Within the semiconductor material 10, electrical current is generated and moves toward the gridlines for collection, as indicated by the current flow lines 18 in FIG. 1. Because current flows toward the gridlines 12, the current density in the semiconductor 10 increases with proximity to the gridlines 12. The gridlines 12 are closely spaced to minimize the resistive power loss (Ploss˜I2R) resulting from the high current density under concentrated sunlight conditions.
The closely-spaced gridlines 12 reduce the resistive power loss, but, since the gridlines 12 are optically opaque, they prevent light from reaching a portion of the semiconductor surface 14. Typically the gridlines 12 cover 5-10% of the total surface area. The most advantageous spacing between gridlines is therefore influenced by the trade-off between resistive power loss and the obscuration (shadowing) loss. An increase in cell efficiency can be achieved if the metal gridline spacing can be increased.
At the high concentrations (>500×) used for CPV systems, obscuration by the metal gridlines limits solar cell performance. The recent world records of over 40% multifunction efficiencies were actually achieved at significantly lower concentrations (<250×). Efficiency falls off at higher concentrations due to obscuration. Therefore, reductions in obscuration can boost efficiency at higher concentrations.
Transparent conductive coatings (TCCs) are sometimes applied to the sunward surfaces of solar cells. TCCs provide electrical conduction (reduced resistance) while allowing light to pass through them to the underlying semiconductor. TCCs are applied in a blanket fashion to the entire sunward surface of each solar cell. Examples of TCCs in use today include transparent conductive oxides (TCOs), such as indium-tin oxide (ITO) or zinc oxide. Optical transmission of TCCs is typically 80-95%.
Many TCCs available today are limited in both their conduction and optical transmission. As such, they are mainly used in a number of lower-cost, lower-efficiency solar cell technologies. TCCs are currently a net benefit only for cells that are both more resistive and less sensitive to optical transmission losses than the high-efficiency cells used in CPV. When used in a high-efficiency multijunction cell, a blanket coat of TCC decreases the resistive power losses, but introduces optical transmission losses that more than offset the gains.